The present invention generally relates to making a thin photoresist layer having a low defect density. In particular, the present invention relates to reducing, the occurrence of pinholes in thin photoresist layers.
Microlithography processes for making miniaturized electronic components, such as in the fabrication of computer chips and integrated circuits, involve using photoresists. Generally, a coating or film of a photoresist is applied to a substrate material, such as a silicon wafer used for making integrated circuits. The substrate may contain any number of layers or devices thereon. The photoresist coated substrate is baked to evaporate any solvent in the photoresist composition and to fix the photoresist coating onto the substrate. The baked coated surface of the substrate is next subjected to selective radiation; that is, an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the photoresist coated surface. Types of radiation commonly used in microlithographic processes include visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy. After selective exposure, the photoresist coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
As the trend toward smaller and smaller semiconductor device dimensions continues, there is a constant demand to improve the methods of fabricating and processing such devices. For example, improvements in lithography techniques lead to thinner gates, smaller vias, thinner lines and higher density devices among other desirable features. Lithography techniques are governed by a number of factors including precision, repeatability, alignment, resolution, uniformity, and the like. Although there are many factors associated with lithography techniques, much attention is devoted to improving resolution.
Resolution is affected, in part, by the thickness of a photoresist coating or layer. Swelling problems, due to interactions between the photoresist and developer in some instances, can be minimized by reducing the thickness of the photoresist. The deleterious effects of electron scattering are also minimized by reducing the thickness of the photoresist. Primarily though, reducing the resist thickness improves the lithography process depth of focus, and thus critical dimension control can be improved using relatively thin photoresists. Therefore, as the thickness of the photoresist layer is reduced, resolution is improved. However, there are limitations associated with making thin photoresist layers. This is because various problems can arise with making thin photoresist layers. One particular problem is defect density or the occurrence of pinholes in thin photoresist layers. Since remediation of pinholes in photoresists is generally not possible, pinholes in photoresist layers constitute unrepairable defects. It is therefore desirable to provide a thin photoresist layer having a low defect density.
Substantial photoresist thickness variations are also frequently encountered when attempts are made to coat a thin photoresist layer over a nonplanar surface, such as a semiconductor substrate having any number of gates or channels therein. 1he thickness variations in the photoresist layer cause various problems including nonuniform exposure and/or curing, variations in development rate of the exposed photoresist, uneven line width and non-vertical patterned side walls. It is therefore desirable to provide a thin photoresist layer having a minimal thickness variations.
The present invention provides relatively thin photoresist layers having better surface planarity and/or low defect densities. The present invention also provides methods of reducing pinholes in photoresists. As a result of the present invention, thinner photoresist layers can be used which enables improved critical dimension control during lithography or higher yields.
In one embodiment, the present invention relates to a method of forming a thin photoresist layer having a low defect density, involving the steps of depositing a photoresist layer having a thickness from greater than about 0.5 xcexcm to about 2 xcexcm on a semiconductor substrate; and removing at least a portion of the photoresist layer to provide the thin photoresist layer having the low defect density and a thickness from about 0.1 xcexcm to about 0.5 xcexcm.
In another embodiment, the present invention relates to a method of reducing pinhole defects in a thin photoresist layer having a thickness below about 0.5 xcexcm comprising the steps of depositing a layer of the photoresist material having a thickness greater than about 0.5 xcexcm; and etching at least a portion of the photoresist material to provide the thin photoresist layer having the thickness below about 0.5 xcexcm, wherein the thickness of the thin photoresist layer is about 90% or less than the thickness of the original layer of the photoresist material.
In yet another embodiment, the present invention relates to a method of processing a semiconductor substrate, involving the steps of providing a semiconductor substrate having a nonplanar surface; optionally depositing an antireflection layer over the semiconductor substrate; spin-coating a photoresist layer having a thickness from about 0.5 xcexcm to about 2 xcexcm on the optionally antireflection layer coated semiconductor substrate; and etching the photoresist layer to provide a thin photoresist layer having a thickness from about 0.1 xcexcm to about 0.5 xcexcm, wherein the thickness of the thin photoresist layer is smaller than the thickness of the spin-coated photoresist layer.